1. Field of the Invention
The present invention relates to an exhaust gas cleaning device having a particulate filter for an internal combustion engine.
2. Description of Related Art
Exhaust particulate matter (PM), which is contained in exhaust gas of a diesel engine, is considered as a major environmental issue, and it has been proposed to install a diesel particulate filter (DPF) for collecting the PM in the diesel engine. The DPF is periodically regenerated by combusting and removing the collected PM to recover its performance for collecting the PM. The combustion of the PM normally requires the sufficiently high temperature of the DPF. However, in some cases, the combustion of the PM is carried out using the oxidation reaction heat generated by the oxidation catalyst. One DPF system (hereinafter referred to as an upstream catalyst type DPF system) has the oxidation catalyst, which is arranged on the upstream side of the DPF. Another DPF system (hereinafter referred to as a single DPF system) includes the DPF alone, and the catalyst is held by a filter substrate of the DPF.
In order to increase the temperature of the DPF, the temperature of the exhaust gas, which is exhausted from the engine, may be increased by throttling of the intake air, retarding of the fuel injection or increasing of the exhaust gas recirculation (EGR). Alternatively, the uncombusted component (hereinafter, also referred to as the uncombusted hydrocarbon, i.e., the uncombusted HC), which is contained in the exhaust gas exhausted from the engine, may be intentionally increased by, for example, post fuel injection to generate the catalyst reaction heat (see Japanese Unexamined Patent Publication No. 2003-172185). In each of the above cases, the energy, which is not converted into the engine power, is wasted, so that the fuel consumption needs to be concerned. When the temperature of the DPF is sufficiently high, the combustion speed is accelerated. Thus, the time required for regeneration of the DPF is shortened to advantageously reduce the fuel consumption. However, when the temperature of the DPF is rapidly increased, the DPF may be damaged. Thus, the temperature suitable for the regeneration should be used as the target temperature to maintain the temperature of the DPF.
However, with respect to the above two methods for increasing the temperature of the DPF, the one method, which increases the temperature of the exhaust gas and supplies the exhaust gas to the DPF, results in the loss of energy, which is lost through the engine or the exhaust pipe. Thus, the other method, which increases the amount of the uncombusted HC, is more advantageous in terms of the fuel consumption over the above one method. Furthermore, the single DPF system is more advantageous over the upstream catalyst type DPF system since the single DPF system can be constructed only from the DPF, thereby allowing the low manufacturing costs and the low weight.
However, in the single DPF system, when the method of increasing the uncombusted HC is used, the following disadvantageous point exists. That is, in the catalyst upstream type DPF system, the exhaust gas, which has the increased temperature due the catalytic reaction heat, is supplied to the DPF. In contrast, in the single DPF system, the catalytic reaction of the uncombusted HC occurs upon entering of the uncombusted HC in the DPF. FIG. 16 shows a relationship between the temperature of the DPF and the reaction speed for oxidizing the HC. When the temperature of the DPF is increased, the activation level of the catalyst is increased to accelerate the reaction speed. However, when the temperature of the DPF is below a predetermined temperature, the catalyst is not activated effectively, so that the HC cannot be sufficiently combusted. The temperature of the DPF needs to be equal to or higher than 600 degree Celsius to achieve the stable effective combustion of the PM. When the large amount of uncombusted HC is exhausted from the engine at the low temperature of the exhaust gas, the unreacted HC may adhere to the catalyst to cause poisoning of the catalyst. Thus, in the case of the single DPF system, the temperature increase, which is caused by the catalyst reaction of the uncombusted HC, does not substantially occur at or near the front end surface of the DPF, which is the upstream end of the DPF. Thus, there exists the temperate gradient, in which the temperature increases from the upstream end of the DPF toward the downstream end of the DPF.
As shown in FIG. 17, in the case where the exhaust gas temperature at the entry of the DPF is equal to or less than the predetermined temperature (hereinafter, referred to as the regenerative temperature), above which the amount of combusted PM is greater than the amount of exhaust PM outputted from the engine, and the temperature gradient is held steady, the temperature at the upstream end of the DPF will not become equal to or greater than the regenerative temperature. Thus, the collected PM, which is collected at the upstream end of the DPF, cannot be sufficiently combusted and cannot be sufficiently removed. As a result, the upstream end of the DPF may be clogged by the PM. With reference to FIG. 18, the above state may be encountered in the low load range of the engine, which occurs at the time of the idling state of the engine or of the low traveling speed of the vehicle where the exhaust gas temperature cannot be increased to the regenerative temperature.
Thus, in that operational range of the engine, the temperature of the exhaust gas, which is outputted from the engine, should be increased to make the temperature of the exhaust gas at the entry to the DPF equal to or greater than the regenerative temperature to avoid clogging of the DPF and/or the poisoning of the catalyst of the DPF. However, the method for increasing the exhaust gas temperature by, for example, throttling the intake air, is limited to a range, which does not cause misfiring. Thus, the temperature increase is limited to the unsatisfactory level.